The present invention relates to a smokable synthetic food-packaging film which shows a smoke permeability and an oxygen-barrier property, and which also has excellent smokability and preserving effect without secondary-packaging in plastic bags, and more particularly, to a smokable food-packaging film comprising:
at least one layer of a mixture of 50 to 80 wt% polyamide having a melting point of 120.degree. to 210.degree. C. and a recrystallization temperature of not lower than 50.degree. C. and 20 to 50 wt% .alpha.-olefin-vinyl alcohol copolymer, said film having properties of an initial stress of not higher than 0.6 Kg/mm.sup.2 at a 25% elongation, a transmission to 50% concentration of methanol of not less than 200 g/m.sup.2.day.atm at a temperature of 60.degree. C. and a relative humidity of 0%, and an oxygen gas-transmission of not more than 50 cc/m.sup.2.day.atm at a temperature of 30.degree. C. and a relative humidity of 60%, and the thickness of said film being 20 to 60 .mu.m. PA0 a. forming a blend of about 90 to 20 weight percent of a film-forming polyamide resin having a molecular weight of about 20,000 to 30,000 and a melting point temperature of about 415 degrees to 440 degrees Fahrenheit, conversely about 10 to 80 weight percent of an ethylene vinyl alcohol copolymer having a molecular weight of about 15,000 to 30,000 and a melting point temperature of about 325 degrees to 375 degrees Fahrenheit, and about 2 to 25 percent, based upon the weight of said polyamide resin, of a plasticizer therefor, said plasticizer being effective to lower the processing temperature of said polyamide resin to a value approximately that of said copolymer and below the temperature at which any substantial amount of degradation would occur therein under appropriate conditions for extrusion; PA0 b. heating said blend to said processing temperature; PA0 c. forcing said blend through an extrusion die to produce a film of said blend; and PA0 d. cooling said film to obtain a product having such desirable levels of toughness and oxygen barrier capability. PA0 at least one layer of a mixture of 50 to 80 wt% polyamide having a melting point of 120.degree. to 210.degree. C. and a recrystallization temperature of not lower than 50.degree. C. and 20 to 50 wt% .alpha.-olefin-vinyl alcohol copolymer, said film having properties of an initial stress of not higher than 0.6 Kg/mm.sup.2 at a 25% elongation, a transmission to 50% concentration of methanol of not less than 200 g/m.sup.2.day.atm at a temperature of 60.degree. C. and a relative humidity of 0%, and an oxygen gas-transmission of not more than 50 cc/m.sup.2.day.atm at a temperature of 30.degree. C. and a relative humidity of 60%, and the thickness of said film being 20 to 60 .mu.m. PA0 a layer of a mixture of 50 to 80 wt% polyamide having a crystal melting point of 120.degree. to 210.degree. C. and a recrystallized temperature of not lower than 50.degree. C. and 20 to 50 wt% of an .alpha.-olefin-vinyl alcohol copolymer; and PA0 at least one layer of a material selected from the group consisting of (a) an .alpha.-olefin copolymer of polyethylene, polypropylene or polybutene-1, (b) a copolymer of ethylene, propylene or butene-1 and vinyl acetate or methacrylic ester, (c) a copolymer of ethylene, propylene or butene-1 and methacrylic acid, or a metal salt thereof, (d) an elastomer of a plasticized polyvinyl chloride, polyester or a styrene rubber, (e) a cellulose and (f) a polyamide, said film having properties of an initial stress of not higher than 0.6 Kg/mm.sup.2 at a 25% elongation, a transmission to 50% concentration of methanol of not less than 200 g/m.sup.2.day.atm at a temperature of 60.degree. C. and a relative humidity of 0%, and an oxygen gas-transmission of not more than 50 cc/m.sup.2.day.atm at a temperature of 30.degree. C. and a relative humidity of 60%, and the thickness of the layer of said mixture being 20 to 60 .mu.m and the thickness of the layer of said thermoplastic material being 10 to 50 .mu.m.
Hitherto, the packages for foodstuffs such as prepared meat which is to be smoked is mainly conducted by primary packaging and secondary packaging. For primary packaging, a packaging material is selected so that it is suitable for forming and maintaining the shape of foodstuffs which are packed directly into the packaging material, and for rapidly permeating smoke components during the smoking process. As examples of packaging materials used for primary packaging, casings made from animal intestines such as those of cows, pigs, or sheep, and cellulose casings made of viscose film may be mentioned. These smokable primary packaging materials, however, lack gas-barrier properties and are unsuitable as packaging materials for ordinary food-storage. Therefore, it is necessary to package the foodstuffs again in a film which has a gas-barrier property (secondary packaging).
Other known smokable primary packaging materials are films of natural polymer such as collagens, chitins and polysaccharides, or paper which is impregnated with a polyvinyl alcohol resin [Japanese Patent Publication No. 47-43198 (1972)], paper impregnated with an ethylene-vinyl alcohol copolymer containing a plasticizer [Japanese patent application Laid-Open (KOKAI) No. 52-57347 (1977)], and polyester copolymer films containing polyalkylene oxide [Japanese Patent Publication No. 59-117530 (1984)]. These smokable primary packaging materials also have insufficient gas-barrier property, and thus necessitate a secondary packaging for the preservation of the food. Although films which are mechanically perforated or porous films have been proposed in order to improve the smoking properties (British Pat. No. 1,397,472), these packaging materials also necessitate secondary packaging. Thus, conventional smokable packaging materials required a secondary packaging process, and as a result there are defects of the increases of the number of manufacturing steps, packaging materials, and packaging machines, and thus increase the manufacturing cost.
As known films having both smoke-permeability and oxygen gas-barrier property, there are films of polyamides such as polycaprolactam and of polymer blends of polyamide and at least one of ionomer resin, modified ethylene-vinyl acetate copolymer and modified polyolefin (European patent application No. 0,139,888A1).
European patent application No. 0,139,888A1 suggests that films consisting of polyamides which can absorb at least 3 wt% of their own weight of water up to saturation point, such as, for example, polycaprolactam, polyaminooenanthic acid amide, polyhexamethylene adipamide and polyhexamethylene sebacamide, and films consisting of polymer blends of polyamides such as those described above, and at least one of ionomer resin, modified ethylene-vinyl acetate copolymer and modified polyolefin, are useful as smokable thermoplastic synthetic casings. However, the only films that are concretely disclosed therein are transparent, colorless shrinkable casings formed of polycaprolactam or polyhexamethylene adipamide. The casings of polycaprolactam and polyhexamethylene adipamide show an insufficient oxygen gas-barrier property, and the casings thereof are therefore inadequate for the prolonged storage periods of foods, in particular, for foodstuffs which are susceptible to oxygen.
Further, a food-packaging film comprising polyamide and ethylene-vinyl alcohol copolymer is disclosed in the following U.S. Patents and German patent application.
U.S. Pat. No. 4,347,332 discloses a method for the production of a film having relatively high levels of strength and toughness and good oxygen barrier properties, comprising:
U.S. Pat. No. 4,427,825 discloses a heterogeneous melt blended composition consisting essentially of: a polyamide, and from 1 to 65% by weight of an ethylene vinyl alcohol copolymer having 15 to 65 mol percent ethylene, in the absence of plasticizer, wherein there are regions of ethylene vinyl alcohol copolymer having an average diameter of less than 500 angstroms in the polyamide, said composition having been processed at a temperature of from about 225.degree. C. to 252.degree. C. and being substantially free from crosslinking.
U.S. Pat. No. 4,468,427 discloses a composition of matter comprising a polyamide and from one to 65 percent by weight of an ethylene vinyl alcohol copolymer. A preferred composition comprises polyepsiloncaprolactam and from 5 percent to 30 percent by weight of ethylene vinyl alcohol copolymer containing 15 to 65 mol percent ethylene.
German patent application No. 3229158 discloses a process for producing a film from a composition of a copolymer of ethylene and vinyl acetate and Nylon, which process comprises the steps of (a) preparing a composition comprising about 90 to about 10% by weight of a Nylon resin, about 10 to about 90% by weight of a copolymer of ethylene and vinyl alcohol of a molecular weight of about 15,000 to about 30,000 and a melting point of about 325.degree. to 375.degree. F., and a plasticizer of upto 25% by weight based on the weight of the Nylon resin, the processing temperature of the composition being close to the processing temperature of the copolymer and being lower than the limiting vlaue at which the deterioration of the substantial amount occurs to the copolymer under the appropriate extruding conditions, (b) heating the thus obtained composition to the processing temperature, (c) extruding the composition through an extruding die to produce a film, (d) adjusting the temperature of the thus extruded film in order to obtain an effective orientation of the film, (e) stretching the thus adjusted film at a temperature at which at least a monoaxial orientation is obtained, and (f) cooling the thus stretched film, thereby obtaining the relatively thin (in thickness) film which has the strength of a desirable level, the toughness and the oxygen gas-barrier property.
As a result of various studies into how to achieve this aim, it has been found by the present inventors that the smoke-permeability of a food-packaging film during a smoking process has a close relationship with its transmission to methanol in the smoke components, and that, in the case of a foodstuff which is susceptible to oxygen is packaged with a smokable food-packaging film of a thermoplastic resin which has oxygen gas-barrier property sufficient for practical use and also a predetermined transmission to methanol, on the assumption that its smoke-permeability is the same as transmission to methanol, a food package which has good smokability and which can store for long time is obtained without secondary packaging in plastic bags. The present invention has been attained on the basis of this findings.
Accordingly, it is an object of the present invention to provide a food-packaging film of a thermoplastic resin which has good smokability and excellent oxygen gas-barrier property.
Namely, it is an object of the present invention to provide a smokable food-packaging film which shows such good smoking effect and oxygen gas-barrier property, and can be used to preserve foodstuffs which are susceptible to oxygen for long periods.